Gas-phase polymerization processes are economical processes for the polymerization of olefins such as homopolymerizing ethylene or propylene or copolymerizing ethylene or propylene with other olefins. Suitable reactors for carrying out such gas-phase polymerizations are for example fluidized-bed reactors, stirred gas-phase reactors or multizone circulating reactors with two distinct interconnected gas-phase polymerization zones. These processes are usually carried out in a gas phase comprising monomers and comonomers and additionally also often other gaseous components such as polymerization diluents, for example nitrogen or alkanes, or hydrogen as molecular weight modifier or low-molecular weight reaction products. The obtained products are generally solid polyolefin particles which are formed by polymerization catalyst systems usually comprising particulate catalyst solids.
When the produced material is continuously or discontinuously removed from the gas-phase polymerization reactors, the discharged product is not only pure polyolefin but contains also portions of the gaseous phase as intergranular gas or as dissolved hydrocarbons. For ecological, safety and quality reasons, these entrained parts of the gas-phase have to be removed from the polyolefin particles because its components constitute an impact on the environment, gaseous hydrocarbons may result in the formation of explosive mixtures in downstream equipment and remaining non-polymerized components in the final polyolefin polymers may cause problems of quality such as odor formation. Furthermore, it is desirable to recycle unreacted monomer and comonomer to the polymerization process.
A common practice for removing the entrained parts of the gas-phase from the polyolefin particles is contacting the particles with a stream of an inert gas, usually in countercurrent flow. Such a step is frequently denoted as “degassing” or “purging”. Often such a degassing or purging step is combined with a step of deactivating the polymerization catalyst and/or cocatalysts, e.g. by reacting the catalyst and/or cocatalysts with water.
For examples, EP 339 122 A1 discloses a two-step method for removing unpolymerized gaseous monomers from a solid olefin polymer while deactivating Ziegler-Natta catalysts and organometallic catalyst residues present in said solid olefin polymer, which is carried out in a single vessel. The solid olefin polymer is first countercurrently contacted with a first purge gas, preferably pure nitrogen, in an upper zone of the purge vessel, then transferred in the lower zone of the purge vessel and there countercurrently contacted with a second purge gas containing water, preferably pure nitrogen and steam.
U.S. Pat. No. 5,071,950 refers to a process for the continuous preparation of an ethylene/α-olefin copolymer in which the resulting ethylene copolymers are transferred to a let-down zone of reduced pressure and then the solid copolymer is freed of residual monomers and odor and flavor substances in a two-step way by first flushing with gaseous ethylene and then flushing with a mixture of nitrogen and steam. Similarly, EP 683 176 A1 describes a process for continuously manufacturing ethylene (co-)polymer in a gaseous phase in which the solid (co)polymer, after having passed a depressurization zone, is subjected to (1) a non-deactivating flushing with respect to the active catalytic residues, and subsequently (2) a deactivating flushing with a gaseous mixture of nitrogen, water and oxygen. Preferably, the gas for the non-deactivating flushing is the gaseous reaction mixture which circulates in the polymerization zone.
WO 2006/082007 A1 discloses an ethylene polymerization process in a gas-phase reactor in which the obtained polymer particles are discharged from the reactor, separated from the major part of the concomitantly discharged reactor gas and thereafter degassed, wherein the degassing is carried out with a propane fraction separated from the concomitantly discharged reactor gas.
WO 2008/015228 A2 describes a process to perform the finishing of polyolefins produced by gas-phase catalytic polymerization of one or more α-olefins in the presence of a polymerization diluent selected from a C3-C5 alkane, in which the polyolefin particles discharged from the gas-phase reactor are subjected to a first degassing step in which the polyolefin particles are countercurrently contacted with a gaseous stream containing at least 85 mol-% of a C3-C5 alkane and then to a second degassing step in which the polyolefin particles are countercurrently contacted with steam. Since the steam partly condensates on contact with the polyolefin particles, the process requires a subsequent drying step.
These processes provide possibilities for a sufficient degassing of polyolefin polymers prepared in gas-phase polymerization. However, they require significant efforts to ensure that a desired low level of residues is reached, as well in operating expenses as in investment costs, especially if the polymerization is carried out with a C3-C5 alkane as polymerization diluent.
Thus, it was the object of the present invention to overcome the disadvantages of the prior art and to find a possibility for degassing polyolefin particles obtained by gas-phase polymerization in the presence of C3-C5 alkane as polymerization diluent, which can be carried out cost-efficiently as well with respect to operating expenses as to investment costs, which allows to recycle almost all monomer and comonomer concurrently discharged with the polyolefin particles and which provides polyolefin products with a sufficiently low level of volatile components.